Wilson Aires Ortiz, Federal University at São Carlos, Brazil

Jeudi 11 Septembre 2014, 14h
Amphi Holweck, Esc C, 1ème etage

Critical current enhancement and guidance of flux avalanches in microstructured superconducting films

Wilson Aires Ortiz
Superconductivity and Magnetism Group
Physics Department, Federal University at São Carlos
São Carlos, SP, Brazil

In real applications of superconducting films, the availability of high critical currents (Jc) is a typicalrequirement, not only for the purpose of transporting large current densities, but most likely because of the need to screen a substantial part of the applied magnetic field. The insertion of arrays of antidots (ADs) in a superconducting film can lead, at temperatures close to the critical temperature, Tc, to an increase of Jc. However, the existence of such pinning centers (PCs) facilitate flux channeling and, for temperatures below a sample-dependent characteristic threshold limit, unwanted instabilities of thermomagnetic origin lead to the occurrence of guided flux avalanches which, in the presence of ADs, occur for a wider interval of the applied magnetic field. A good compromise between the beneficial enhancement of Jc and the unavoidable increase of channeling when lattices of PCs are introduced, can be achieved by insertion of a non-periodic array of ADs, as previously pointed out by Silhanek et alli [1] ; and realized in practice by Motta and coworkers [2], followed by other groups [3,4].
Most recently, our group conducted studies on superconducting systems with homogeneous and inhomogeneous distributions of ADs, as well as some others for which the shape of the ADs, or their separation – or even the symmetry of the lattice – varies from place to place along the film. This methodical study represents an effort to reveal the actual role played by the geometry of individual ADs – and also of the lattice symmetry – on the morphology of flux avalanches, which are facilitated and guided by the arrays of PCs. The study was conducted on films of Nb and amorphous-MoGe, and includes magneto-optical imaging and measurements of the magnetic response in terms of temperature, applied magnetic field, amplitude of the AC excitation field and its frequency. For films decorated with ADs arranged in a regular mesh, avalanches are guided, forming patterns whose morphology is intimately related to the lattice symmetry and the shape of the holes [5]. For specimens decorated with non-periodic arrays of PCs, Jc is substantially enhanced and the avalanches take place at a reduced portion of the magnetic phase diagram [6]. Superconducting films with a small number of millimeter-sized holes (typically 4, symmetrically displayed) were also studied [7], and the role of the tips of the holes on the morphology of secondary avalanches was demonstrated. In such configurations, circular perforations act as stop-holes, avoiding the appearance of secondary avalanches. A model considering the transport of supercurrents and heat transfer (within the sample and to the substrate), accounts for the morphology and time evolution of the avalanches actually observed [8].
On the other hand, the morphology of such avalanches – and of flux penetration in general – is certainly dependent on the occurrence of the so-called current crowding effect [9,10], associated with the fact that the streamlines of bending currents cannot cross each other, thus causing a sort of compression of current lines and, as a consequence, a local inhomogeneity of the critical current, manifested as a bias to vortex entrance and propagation throughout the region where the effect is taking place. Our group has studied this effect on corner shaped microstrips of Al [11], showing that, as predicted [10], sharp bends lead to measurable asymmetries on vortex dynamics.

References

[1] A. V. Silhanek et al., Appl. Phys. Lett. 89, 152507 (2006)
[2] M. Motta et al., Appl. Phys. Lett. 102, 212601 (2013)
[3] S. Guénon et al., Appl. Phys. Lett. 102, 252602 (2013)
[4] Y. L. Wang et al., Phys. Rev. B87, 220501 (2013)
[5] M. Motta et al., unpublished (2014)
[6] M. Motta et al., Appl. Phys. Lett. 102, 212601 (2013)
[7] F. Colauto et al., Appl. Phys. Lett. 103, 032604 (2013)
[8] J. I. Vestgården et al., Phys. Rev. B 85, 014516 (2012)